Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Investments in subsidiaries	$ 712.8	$ 449.0
Other assets	441.3	443.2
Total assets	14,383.9	13,471.8	$ 12,478.7
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	249.0	135.0
Long-term debt	253.6	302.3
Other liabilities	428.2	453.1
Total liabilities	12,576.5	11,681.7
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0.0	0.0
Common stock, $1,000 par value, authorized 75,000,000 shares; issued, 2021, 66,436,821; 2020, 66,316,797	0.1	0.1
Additional paid-in capital	495.3	488.4
Retained earnings	1,524.9	1,434.6
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturity securities	290.7	366.3
Net funded status of benefit plans	(10.2)	(11.2)
Treasury stock, at cost, 2021, 25,043,337 shares; 2020, 24,902,579 shares	(493.4)	(488.1)
Total shareholders' equity	1,807.4	1,790.1	$ 1,567.3
Total liabilities and shareholders' equity	14,383.9	13,471.8
Parent company
ASSETS
Investments and cash	115.3	4.1
Investments in subsidiaries	2,191.3	2,121.0
Other assets	11.3	58.9
Total assets	2,317.9	2,184.0
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	249.0	135.0
Long-term debt	248.6	248.3
Other liabilities	12.9	10.6
Total liabilities	510.5	393.9
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0.0	0.0
Common stock, $1,000 par value, authorized 75,000,000 shares; issued, 2021, 66,436,821; 2020, 66,316,797	0.1	0.1
Additional paid-in capital	495.3	488.4
Retained earnings	1,524.9	1,434.6
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturity securities	290.7	366.3
Net funded status of benefit plans	(10.2)	(11.2)
Treasury stock, at cost, 2021, 25,043,337 shares; 2020, 24,902,579 shares	(493.4)	(488.1)
Total shareholders' equity	1,807.4	1,790.1
Total liabilities and shareholders' equity	$ 2,317.9	$ 2,184.0